Spin forming is the reshaping of a flat or hollow material using a point deformation process that uses the combined forces of rotation and pressure. Spin forming involves spinning the product on a lathe and plastically deforming the product onto a tooling mandrel that rotates with the product. By deforming the product onto the mandrel, the finished product acquires the contours of the mandrel. Thus, a flat metal sheet can be formed to a desired shape.
A single mandrel can be used to spin form many finished products; however, all the finished products possess only the shape of that individual mandrel. Thus, multiple mandrels are required to form products having different shapes and/or sizes. A mandrel can be costly and take a long time to create; therefore, it is desirable to minimize the number of mandrels required to form numerous products of dissimilar shape because of tooling costs and lead times.
Material costs and lead times are also important considerations in the selection and manufacturing of the materials for spin forming. Generally, raw materials having standard dimensions cost less and can be more quickly procured than raw materials that are uniquely dimensioned. Because many spin forming applications require flat metal sheets with unique dimensions, it is desirable to convert metal sheets of standard size to the metal sheets of unique dimensions, prior to the spin forming process, in a cost-effective manner without adversely affecting the material properties. For example, metal sheets that are of a standard size but that are smaller than the unique dimensions that are desired may be joined together to create a metal sheet with the unique dimensions.
Conventional welding techniques are typically used to join metal sheets; however, some metals, such as high strength precipitation strengthened aluminum alloys, cannot be satisfactorily joined by conventional welding techniques. Friction stir welding is one method of joining metal sheets that addresses the difficulties of welding some aluminum alloys or other materials not easily joined by conventional welding techniques. U.S. Pat. No. 5,460,317 to Thomas et al., discloses a method of friction stir welding. Two sheets of material are friction stir welded by butting the two sheets together and then running a rotating probe along the joint line. The rotating probe creates a local region of highly plasticized material, and the plasticized material is swept by the rotating probe, such that the material of the two sheets join and upon cooling create a butt joint. The friction stir welding process can join two metal sheets; however, the material properties along the joint are sufficiently different from the material properties of the other portions of the sheets of material, such that the welded sheet may not satisfy the same engineering criteria of the base material. Therefore, a friction stir welded metal sheet that is subsequently spin formed creates a finished product with different material properties along the original friction stir weld joint.
Therefore, a need exists for a spin forming mandrel that provides the ability to spin form metal sheets into multiple shapes and/or sizes. Further, a need exists to utilize metal sheets of standard dimensions, that have been joined prior to the spin forming process, in order to create a larger metal sheet with unique dimensions, but without reducing the material properties of the finished product, such as along a weld joint.